Near-infrared spectroscopy (NIRS) is a potentially effective technique for non-invasive functional studies of the human brain. The strengths of this technique are its high sensitivity to cerebral oxy- and deoxy-hemoglobin concentrations ([HbO2] and [Hb], respectively), and therefore to the cerebral hemodynamics, and its high temporal resolution in the 10 ms range. While NIRS has yielded very promising results in functional brain studies, there are a number of open questions that await an answer before NIRS can be successfully applied as a clinical tool. For instance, what is the role played by the baseline hemodynamics in the findings the 1 the activated cortex area is better localized in [Hb] maps than in [HbO2] maps, and 2 the cortex hemoglobin saturation increases substantially upon activation? What is the physiological origin of the highly fluctuating baseline optical signals? Can these baseline cerebral signals be exploited to extract physiological and potentially diagnostic parameters? Furthermore, can NIRS non-invasively detect the fast neural activation, occurring with a latency of 50-100 ms? This proposal aims at addressing these issues with the goal of characterizing the temporal-spatial features of the baseline cerebral hemodynamics and of the near-infrared response evoked by cerebral activation. To achieve this goal, we will perform spatially- resolved, frequency-domain spectral measurements (in the wavelength range 630-850 nm) over large brain areas of healthy subjects at rest, and during visual or motor stimulation. Simultaneously with the optical measurements, we will continuously monitor systemic physiological parameters such as the heart rate, the arterial saturation, the breathing temporal pattern, and the mean arterial blood pressure. This research constitutes a first step toward the realization of innovative clinical tools for the continuous monitoring of cerebral oxygenation in infants, and for the diagnosis and follow-up of cerebrovascular diseases and psychiatric syndromes.